Burners are used to provide heat in a variety of industrial applications. In general, burners employ a conventional nozzle to mix fuel and air and thereby create a combustible mixture.
Unfortunately, conventional nozzles do not cleanly and completely burn the air-fuel mixture over the entire operational range of the burner. Burners are generally capable of operating over a specified range of air and fuel inputs Conventional nozzles typically achieve optimum mixture over a limited range of air and fuel inputs. Within the preferred range, mixture of the fuel and air is complete and the burner cleanly burns the mixture. Operation of conventional burners with air and fuel inputs outside of the preferred range, however, causes poorer quality mixing of fuel and air by the nozzle. At certain inputs, the nozzle does not mix the air and fuel completely, causing less clean combustion which thereby increases the amount of combustion byproducts such as carbon monoxide and hydrocarbons. At other inputs, the air flow through the nozzle quenches the flame, thereby reducing the temperature of the flame which leads to incomplete combustion.
Conventional nozzles also have difficulty igniting a flame at low fire. To ignite a flame, the nozzle must thoroughly mix the air and fuel to obtain a combustible mixture. At low input levels, conventional nozzles fail to consistently provide a sufficient air-fuel mixture, thereby reducing the probability of igniting a flame. To address this problem, burners using conventional nozzles often incorporate a separate pilot which provides a continuous support flame.
In some applications, it is desirable to have only a portion of the total combustion take place immediately at the nozzle, with the remainder of combustion taking place downstream of the combustion chamber. In such applications, a certain amount of combustible mixture is needed at the burner nozzle to maintain flame. Air and fuel not combusting immediately at the nozzle flows downstream of the nozzle to mix and combust at a point downstream of the burner. In this manner, a greater amount of heat is delivered to the downstream location for a given size burner.
Unfortunately, burners using conventional nozzles do not adequately control the amount of combustion taking place downstream of the combustion chamber. The air hole pattern of a conventional burner nozzle has air holes spaced throughout the entire radius of the nozzle. As a result, it is difficult to determine which holes are associated with immediate mixing near the nozzle and which holes provide mixing downstream of the nozzle. Control over immediate and downstream mixing is particularly advantageous in burner applications, such as immersion tube burners, which require a portion of the combustion to take place downstream of the nozzle.
Nozzles used in immersion tube burners have a particular problem producing stable combustion at high firing rates when first ignited due to high back pressures inherent in such systems. It is often desirable to run a burner at or near its heating capacity immediately upon start-up. The conventional nozzles of these burners, however, have difficulty maintaining a stable flame at higher firing rates when first ignited. One approach to this problem has been to provide a pilot assembly which warms the combustion chamber before running at high fire. The pilot assembly, however, wastes time and money since the user must wait for the pilot to heat the combustion chamber before running the burner at high fire.